The present invention relates to data transmission systems and, in particular, to channel coding in such systems.
Much attention has been focused in recent years on channel codes which provide so-called coding gain. Prominent among these are the so-called "trellis" codes described in such papers as G. Ungerboeck, "Channel Coding with Multilevel/Phase Signals," IEEE Trans. Information Theory, IT-28, 1982, pages 55-67; A. R. Calderbank and N. J. A. Sloane, "A New Family of Codes for Dial-Up Voice Lines," Proc. IEEE Global Telecomm. Conf., November 1984, pages 20.2.1-20.2.4; A. R. Calderbank and N. J. A. Sloane, "Four-Dimensional Modulation With an Eight-State Trellis Code," AT&T Technical Journal, Vol. 64, No. 5, May-June 1985, pages 1005-1018; A. R. Calderbank and N. J. A. Sloane, "An Eight-Dimensional Trellis Code," Proc. IEEE, Vol. 74, No. 5, May 1986, pages 757-759; and L. F. Wei, "Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space--Part I: 180 Degrees and Part II: Nonlinear Codes," IEEE J. Select. Areas Commun., Vol. SAC-2, September 1984, pages 659-686, all of which are hereby incorporated by reference. Commercial use of these codes has, for the most part, been concentrated in voiceband data sets and other carrier data communications systems.
Moreover, it may be advantageous in such systems to have a spectral null in the passband signal at, for example, the carrier frequency so as to minimize the data signal energy concentration around that frequency.
As an example of this, consider the fact that data transmitted over a communication channel in a passband is often corrupted in the channel by such impairments as Gaussian noise, phase jitter, frequency offset and intersymbol interference. At a receiver, in order to substantially compensate for these channel impairments, and to thereby better recover the transmitted data from the received data signals, the channel characteristics need to be determined. One way of achieving this in some communication areas is to transmit a tone signal, for example at a carrier frequency, along with the data signals over the communication channel. The channel characterization is accomplished by analyzing the received corrupted tone signal with reference to the transmitted version.
However, a problem related to this characterization method is that the received tone signal cannot often be substantially isolated from the data signal transmitted therewith. The isolation is normally achieved by a band-pass filter operating on a limited band around the carrier frequency. The bandpass filtered version of the received tone signal normally incorporates therein substantial energy contributed from the data signal within that limited band. This unwanted data signal energy contribution is deemed noise which affects the accuracy in characterizing the channel.
The creation of spectral nulls in a line signal using trellis codes has been disclosed in "Trellis Codes With Spectral Nulls," Application Ser. No. 914,337, filed on October 2, 1986, now U.S. Pat. No. 4,831,635, issued May 16, 1989 to T. A. Lee and A. R. Calderbank, which is also hereby incorporated by reference. However, this reference concentrates on a baseband trellis coding rendering one or more spectral nulls in a baseband line signal, rather than a passband signal as of interest here.
Nevertheless, based on the above-mentioned disclosure of Lee and Calderbank, one can create a spectral null in a passband signal at, for example, the carrier frequency by means of conventional modulation techniques. One of these techniques is quadrature amplitude modulation (QAM) which is commonly used in prior art because of the relatively high bandwidth efficiency resulting therefrom. However, in a prior art arrangement, applying the aforementioned baseband trellis coding technique, as referenced, in co-operation with the QAM technique typically requires two independent encoders. The operations of the latter involve independently taking in first and second data words respectively from a data source, encoding the data words in a trellis code and generating first and second multi-dimensional signal points associated therewith. Components of the first (second) signal points are transmitted as the in-phase (quadrature-phase) components of successive two-dimensional signal pulses. This prior art arrangement further requires that the running values of the component sums of the first and second signal points be individually bound.